I would say Wordpress is more robust than tumblr or blogger. It allows for expansion should the need arise. Customization options are greater and easy to implement. I also find the privacy and security settings more extensive and flexible. Yes, between the three I would say Wordpress. I have only read about posterous but it seems what it offers can be accomplished with Wordpress through the use of plug-ins, which is another Wordpress feature that puts it over the top for me.